A photoelectric conversion device is used for detection of an electromagnetic wave and a photoelectric conversion device having sensitivity in ultraviolet rays to infrared light rays is also referred to as a light sensor in general. Among light sensors, one having sensitivity in a visible light region of a wavelength of 400 to 700 nm is referred to as a visible light sensor, which is variously used for an appliance that needs illuminance adjustment or on-off control depending on the environment (see Patent Document 1: Japanese Published Patent Application No. 2005-129909).
In a display device, ambient brightness is detected to adjust display luminance of the display device. This is because it is possible to reduce power consumption and to improve visibility by detecting the ambient brightness and obtaining appropriate display luminance. For example, such a light sensor for adjusting luminance is used in a display portion of a mobile phone and in a display portion of a personal computer. In the mobile phone, power consumption of key button illumination may be reduced as well as power consumption of a display portion.
In addition, luminance of a display portion can be adjusted by detecting luminance of a display device by a light sensor, as well as the ambient brightness. In specific, luminance of a display screen is adjusted with use of a light sensor detecting luminance of a backlight of a liquid crystal display device.
Further, in a display device provided with a projector, convergence adjustment is carried out with use of a light sensor. The convergence adjustment is to adjust an image so that an image of each color of RGB does not generate discrepancy. With use of a light sensor, a location of an image of each color is detected, and the image is located in the right location.
A photoelectric conversion element (a photodiode) is roughly classified into four types: a pn type, a pin type, a Schottky type, and an avalanche type. A pn photodiode is a photoelectric conversion element which uses a junction of a p-type semiconductor and an n-type semiconductor. A pin photodiode is a photoelectric conversion element having a structure in which an intrinsic semiconductor (i-type semiconductor) is interposed between a p-type semiconductor and an n-type semiconductor in a pn photodiode. A pn photodiode has small dark current and a low response speed. A pin photodiode has a high response speed and large dark current.
A p-type semiconductor is a semiconductor in which holes are mainly used as a carrier for transferring charge because of loss of electrons. An n-type semiconductor is a semiconductor in which electrons are mainly used as a carrier for transferring charge because of excessive electrons. An intrinsic semiconductor is a semiconductor formed of a high purity semiconductor material. A Schottky photodiode is a photoelectric conversion element which uses a junction of a gold thin film layer, instead of a p-type semiconductor layer, and an n-type layer. An avalanche photodiode is a photoelectric conversion element with high speed and high sensitivity, in which photoelectric current is doubled by reverse bias voltage applied thereto. Note that a p-type semiconductor layer is referred to as a p-layer, an i-type semiconductor layer is referred to as an i-layer, and an n-type semiconductor layer is referred to as an n-layer.